1. Field of the Invention
The invention relates generally to multilayer conductive polymer films comprising a poly(3,4-ethylenedioxythiophene).
2. Description of the Prior Art
Conducting polymers can be used in organic light-emitting diodes (OLED's). OLED's are an attractive alternative to liquid crystal display technology because they can yield displays that are brighter, lower cost, consume less power, and are lightweight. Other examples of applications for conducting polymers are anticorrosion coatings and printed circuit board finishes.
Extended π-conjugated conducting oligomers and polymers have unique properties that have impacted diverse technologies, and have resulted in the appearance of new ones. A partial list of current and developing applications includes micro- and nanoscale circuitry, throwaway electronic devices such as plastic electrochromic displays, lightweight storage batteries, corrosion protection coatings, antistatic coatings, bio- and chemical sensors, and military applications such as microwave-absorbing materials. In all of these applications a high degree of polymer transparency in visible wavelengths is either necessary or could represent an additional advantageous trait.
Key properties of π-conjugated conducting oligomers and polymers such as bandgap, dielectric constant, and oxidation potential can be varied over much wider ranges than those of other transparent inorganic conductors such as indium tin oxide (ITO) ceramic. This is because of the vast diversity inherent to the organic chemistry of π-conjugated monomers. Other advantages over metals and inorganics include greater plasticity and elasticity, lower mass density, lower coefficient of thermal expansion, greater resistance to chemicals and corrosion, electrochromism, and enhanced power storage capabilities.
As an example of a specific application wherein a highly transparent conducting polymer could have a large impact, one can consider the liquid crystal display devices (LCD's) that are extremely important in current information technology and OLED's under development for next generation displays. In these devices, or in any display device, transparent electrodes are a prime requirement and ITO coated on glass or clear plastic surfaces has generally been used because of its high transparency (˜90%), low surface resistance (˜70 ohms/sq), and high conductivity (˜1000 S/cm). However, the technology is quite expensive and requires high temperature and vacuum treatment. Moreover, the brittleness of the ITO, the non-stoichiometric nature of ITO surfaces, and poor adhesion at the inorganic-organic interface causes serious problems. The deposition of transparent, conductive polymer film on plastic substrates is a highly promising alternative that allows circumvention of these problems. FIGS. 1 and 2 schematically illustrate embodiments of a LCD (FIG. 1) and an OLED (FIG. 2).
Because conducting oligomers/polymers are highly conjugated, they may be colored both in the neutral undoped (non-conducting) state as well as in the cationic, doped (conducting) state. The development of highly transparent conducting polymer thin films has therefore been challenging. Prior art has centered on three families of conducting polymers, polyaniline (PANI), polypyrrole (PPY), and poly(3,4 alkylenethiophene) (PATP).
Jonas et al., U.S. Pat. No. 5,035,926 discloses single layer coatings of poly(3,4-dioxythiophene) made by surface polymerization.
U.S. Pat. No. 6,327,070 to Heuer et al. discloses PATP's with alkylidene groups such as ethylene, propylene, and butylene as well as those containing phenyl and tetradecyl moieties yielding films with modest properties when formed via electropolymerization. For example, poly(3,4 ethylenedioxythiophene) films had a conductivity of 8 S/cm with a transparency of 52%.
An attractive attribute of the monomeric alkylidenethiophenes is their low oxidation potential (˜0.4 V relative to Ag/AgCl) that allows use of mild oxidation agents and results in polymer with high chemical stability. The polymers also have a low band gap (1.5–1.6 eV), causing their absorption λmax values to appear at relatively long wavelengths (590 nm for the undoped form and 775 nm for the doped form). The corresponding colors are dark violet and sky blue. The absorption in the doped conducting form is shifted into the infrared region and therefore the polymers become less heavily colored and are more transparent to the human eye. Within this class of conducting polymers, by far the most extensively investigated has been poly(3,4 ethylenedioxythiophene), or PEDOT, the simplest one from the standpoint of chemical structure. FIG. 3 illustrates the reaction scheme for the polymerization of PEDOT into both doped and undoped forms.
A polymerization method that is well suited for monomers with low oxidation potential such as PEDOT utilizes an oxidant, a base, and an alcohol solvent oxidant. At moderately high temperatures (˜100° C.) the polymerization occurs very rapidly. De Leeuw et al., “Electroplating of Conductive Polymers for the Metallization of Insulators,” Synth. Metals, 66(3), 263 discloses that if the reactant-containing solution is spin-coated onto a suitable substrate such as plastic or glass and then heated, highly conducting insoluble sky-blue films are formed. FIG. 4 illustrates the reaction scheme for the polymerization of PEDOT by surface polymerization. See also Kumar et al., “Conducting Poly(3,4-alkylenedioxythiophene) Derivatives as Fast Electrochromics with High-Contrast Ratios,” Chem. Mater., 10(3), 896 and Pei et al., “Electrochromic and Highly Stable Poly(3,4-ethylenedioxythiophene) Switches Between Opaque Blue-black and Transparent Sky Blue”, Polymer, 35(7), 1347–1351.